The number of people using Facebook during May fell in the US, UK, Canada, Norway and Russia, according to new data.

That means the site's growth has slowed for the second month in a row, even as it approaches 700 million users worldwide.

In the US the site lost about 6 million users, from 155.2 million at the start of May to 149.4 million at its end, according to data gathered from Facebook's advertising tool by the site Inside Facebook.

Canada fell by 1.52 million to 16.6 million and the UK, Norway and Russia all saw falls of more than 100,000 users, the site said.

The slowing growth in user numbers may indicate that Facebook has hit the limits of expansion in the countries where it was first successful – and perhaps even that some early adopters from those countries have stopped using it.

According to the data gathered via the Facebook advertising tool, the world's largest social media site added another 11.8 million users in May, though even that was a slowing compared with April, when it added 13.9 million. Both those numbers were slower, though, compared with the 20 million who have typically been added each month over the past year to take the site to 687 million "monthly active" users – who log in at least once per month.

The drops in the US, Canada, UK and elsewhere held its growth back from the typical growth figure. Without new sources of growth, that could limit the site's total audience.

Eric Eldon of Inside Facebook noted that "by the time Facebook reaches around 50% of the total population in a given country (plus or minus, depending on internet access rates in that country), growth generally slows to a halt ... So far, Facebook has been able to make up stalls and losses with big gains in heavily populated developing countries like Mexico, Brazil, India and Indonesia."

He notes that if it is allowed into China, that growth could take off again towards 1 billion users – although he adds that getting inside the country "could both give it access to hundreds of millions of users and compromise its reputation in the US and many other countries around the world".

China's repressive approach to internet use and communication, whereby users inside the country are banned from accessing a wide range of information hosted outside the country with which the Chinese regime disagrees, has led to sites such as Facebook and Twitter being banned from direct access to Chinese net users, who outnumber the total US connected population.

Despite the month-on-month fall, the US still saw a 23% overall growth in users between May 2010 and May 2011 to 155.2 million users, while the UK grew by 10% to 29.9 million in the same period. The US is the largest country in terms of users overall, with Indonesia second with 36.4m, followed by the UK and Turkey.

The fastest-growing countries among the top 25 largest users areas during the year were Brazil and Russia, each up almost fourfold to 17.1 million and 4.6 million users respectively. A number of other countries nearly doubled their users: India (up 160% to 24.9 million), Thailand (up 143% to 9.1 million users), Egypt (up 108% to 7.1 million), Poland (up 130% to 6.1 million) and Peru (up 126% to 5.5 million).